The easiest way to do it is graph it and count.
Answer:
11 yards
Step-by-step explanation:
The volume of a cone formula is:

Where V is the volume,
r is the radius (half of diameter)
h is the height
Given,
h = 15
V = 475.17
We find the radius (r) first by substituting given values:

The radius (r) is 5.5 yards
We need the diameter, which is DOUBLE THE RADIUS, so diameter would be:
Diameter = 5.5 * 2 = 11 yards
Slope intercept form = Y = Mx + B
15 = -3y + 21x
3y = 21x - 15
Y = 21/3 X - 15/3
Y = 7x - 5 = SI Form